ANATOMY
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system in vascular plants, account has been taken only of what seem the principal and more easily interpreted lines Aberrant of descent. Among some of the Pteridophytes forms. (Selaginella), a type of polystely is found which is not directly traceable to the breaking up of a primitive siphonostele by the crowding of leaf-gaps. Among the Cycadofilices also, a great group of extinct forms occupying the borderland between Ferns and Gymnosperms, side by side with monostelic types which furnish a most interesting sequence leading from the protostele so common among modern Pteridophytes to the modified siphonostele characteristic of modern Cycads, Conifers, and Dicotyledons, there are a number of siphonostelic and polystelic types (the Medullosece and allied forms) in which the breaking of the siphonostele does not depend upon the departure of leaf-traces. The structures/presented are often of considerable complexity, and their origin and relationship with the more easily intelligible types are frequently obscure. In the blade of a typical leaf of a vascular plant—essentially a thin plate of assimilating tissue—the vascular system takes the form of a number of separate, usually branching and Stelar strands. These with their associated tissue of anastomosing stereom form a kind of framework which is of great leaf, root, importance in supporting the mesophyll; but also, and etc. chiefly, they provide a number of channels, penetrating every part of the leaf, along which water and dissolved salts are conveyed to, and elaborated food-substances from, the mesophyll cells. Each bundle is a branch of an original meristele that left the cylinder of the stem. It is always collateral (the phloem being turned downwards and the xylem upwards), even in Ferns, where the meristele or meristeles of the petiole (“ petiolar steles ’ ) are concentric, and it has the ordinary mesodesm and peridesmof the collateral bundle. The latter is often sclerized, especially opposite the phloem, and to a less extent opposite the xylem, as in the stem. As a bundle is traced towards its blind termination in the mesophyll the peridesmic stereom first disappears, the sievetubes of the phloem are replaced by narrow elongated parenchyma cells, which soon die out, and the bundle ends with a strand of tracheids covered by the phloeotermic sheath. The structure of the stele of the primary root as it is found in most Pteridophytes and many Phanerogams has been already described (Fig. 2). The radial structure is characteristic of all root-steles, which have in essential points a remarkably uniform structure throughout the vascular plants, a fact which is no doubt largely dependent on the very uniform conditions under which they live. The larger root-steles are usually polyarch, and seldom have the centre filled up with xylem, this being replaced by a large-celled pith so that the siphonostelic structure is acquired (Fig. 9). Sometimes, however, the centre of a bulky root stele has strands of metaxylem (to which may be added strands of metaphloem) scattered through it, the interstices being filled with conjunctive. The conjunctive of a root-steie possessing a pith is often sclerized between the pith and the pericycle. Sometimes all the parenchyma within the stele undergoes this change. In the roots of some palms the siphonostele breaks up into arcs, which may each acquire a ‘ ‘ stelar ’ structure by the bending in and joining of the edges. For “ laticiferous tissue,” see Cytology. The body of a vascular plant is developed in the first place by repeated division of the fertilized egg and the Develo = growth of the products of division. The body meat of thus formed is called the embryo, and this primary develops into the adult plant, not by continued tissue. growth of all its parts as in an animal, but by localization of the regions of cell-division and growth, such a localized region being called a growing-point. This localization takes place first at the two free ends of the primary axis, the descending part of which is the primary root, and the ascending the primary shoot. Later, the axis branches by the formation of new growing-points, and in this way the complex branch system of axes forming the body of the ordinary vascular plant is built up. In the flowering plants the embryo, after developing up to a certain point, stops growing and rests, enclosed within the seed. It is only on germination of the latter that the development of the embryo into the free plant is begun. In the Pteridophytes, on the other hand, development from the egg is continuous. The triple division of tissues is laid down in most cases at a very early period of development—in the flowering plants usually before the resting stage is reached. In many Pteridophytes the
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first leaf is formed very early, and the first vascular strand is developed at its base, usually becoming continuous with the cylinder of the root; the strand of the second leaf is formed in a similar way and runs down to join that ol the first, so that the stem stele is formed by the joined bases of the leal-traces. In other cases, however, a continuous primitive stele is developed, extending from the primary stem to the primary root, the lealtraces arising later. This is correlated with the comparatively late formation and small development ol the first leaves. 'Ihe evidence scarcely admits of a decision as to which ol these methods is to be regarded as primitive in descent. In the seedforming plants {Phanerogams) one or more primary leaves {cotyledons) are already formed in the resting embryo. Incases where the development of the embryo is advanced at the resting period, traces run from the cotyledons and determine the symmetry of the stele of the primitive axis, the upper part of which shows stem - structure, in some respects at least, and is called the hypocotyledonary stem or hypocotyl, while the lower part is the primary root {radicle). On germination of the seed the radicle first grows out, increasing in size as a wdiole, and soon adding to its tissues by cell division at its apical growing-point. The hypoeotyl usually elongates by its cells increasing very greatly both in number and size, so that the cotyledons are raised into the air as the first foliage-leaves. Further growth in length of the stem is thenceforward confined to the apical growing point situated between the cotyledons. In other cases this growing-point becomes active at once, there being little or no elongation of the hypocotyl and the cotyledon or cotyledons remaining in the seed. The structure of the growing-points or apical meristems varies much in different cases. In most Pteridophytes there is a single large apical cell at the end of each stem and root axis. Qrowina^ This usually has the form of a tetrahedron, with its p0ia^Sm base occupying the surface of the body of the axis and its apex pointing towards the interior. In the stem, segments are successively cut off from the sides of the tetrahedron, and by their subsequent division the body of the stem is produced. In the root exactly the same thing occurs, but segments are cut off also from the base of the tetrahedron, and by the division of these the root-cap is formed. In both stem and root early walls separate the cortex from the stele. The epidermis in the stem and the surface layer of the root soon becomes diflerentiated from the underlying tissue. In some Pteridophyte stems the apical cell is wedge-shaped, in others prismatic ; in the latter case segments^ are cut off from the end of the prism turned towards the body of the stem. In other cases, again, a group of two or four prismatic cells takes the place of the apical cell. Segments are then cut oil from the outer sides of these initial cells. In most of the Phanerogams the apical {or primary) meristem, instead of consisting of a single apical cell or a group of initials, is stratified i.e., there is more than one layer of initials. Throughout the Angiqsperms the epidermis of the shoot originates from separate initials, which never divide tangentially, so that the young shoot is covered by a single layer of dividing cells, the dermatogen. Below this are the initials of the cortex and central cylinder. hether these are always in layers which remain separate is not known, but it is certain that in many cases they cannot be distinguished. This, however, may be due to irregularity of division and displacement of the cells by irregular tensions destroying the obvious layered arrangement. In some cases there is a perfectly definite line of separation between the young cylinder {plerome) and. young cortex {periblem), the latter having one or more layers of initials at the actual apex. This clear separation between periblem and plerome is mostly found in plants whose stem-apex forms a naked cone, the leaves being produced relatively late, so that the stele of the young stem is obvious above the youngest leaf-traces. Where the leaves are developed early, they often quite overshadow the actual apex of the stem, and the rapid formation of leaf tissue disturbs the obviousness of, and perhaps actually destroys, the stratified arrangement of the shoot initials. In this case also, the differentiation of leaf-bundles, which typically begins at the base of the leaf and extends upwards into the leaf and downwards into the stem, is the first phenomenon in the development of vascular tissue, and is seen at a higher level than the formation of a stele. The latter is produced (except in cases of complete astely where a cylinder is never formed) after a number of leaf-traces have appeared on ditt'erent sides of the stem so as to form a circle as seen in transverse section, the spaces intervening between adjacent bundles becoming bridged by small-celled tissue closing the cylinder. In this tissue fresh bundles may become differentiated, and what remains of it becomes the rays of the fullyformed stele. Many cases exist which are intermediate between the two extreme types described. In these the stele becomes obvious in transverse section at about the same level as that at which the first leaf-traces are developed. Where a large-cellea pith is developed this often becomes obvious very early, and m some cases it appears to have separate initials situated below those of the hollow vascular cylinder or modified siphonostele.
